As part of TEPCO’s admission that 100% of the unit 3 core has melted and left the reactor vessel, they included this new information seen in the graph below. The graph shows the times of events related to the melted fuel. TEPCO now estimates the reactor vessel failed just after about 7 am on March 14th. The pedestal line shows a very rapid shift from the reactor vessel to the pedestal. Not the gradual transition seen between the core and the lower plenum. It also shows between about 6 am on March 14th and 1 pm on March 14th that the molten fuel mass significantly migrated from the pedestal to the drywell of the containment structure. This transition activity from reactor vessel, to pedestal to drywell happened over a 7 hour time frame. As shown, a portion of the melted fuel remained in the pedestal, the majority of the fuel migrated to the drywell. TEPCO’s graph also appears to indicate that the transition from the reactor vessel to the pedestal may have been rapid rather than gradual over time. This would indicate a rapid all at once bottom head failure of the reactor vessel rather than a slower drip of melted fuel. This impacts other scenarios such as fuel migration or the ability of the core to burn down through the concrete base mat. The modeling software used (MAAP) does have limits including ones in their corium behavior after it leaves the reactor vessel. This could be part of why the lines for the pedestal and drywell activity are so abrupt. The extremely short time frame does support the idea that all of this transition was rather fast and could have included catastrophic events like a rapid bottom head failure or a quick exit of the corium from the pedestal.
The hydrogen and ex-vessel steam explosion at unit 3 happened at 11:01 am March 14th. This is roughly the point on this graph (below) where you see the black upward arrow on the timeline. This puts the explosion roughly in the middle of the melted fuel migrating to the drywell. Since the modeling software has limits the actual bottom head failure or the loss of the bulk of the melted fuel could have happened closer to the time of the explosion.
One working theory has been that the containment vessel if partially filled with water (TEPCO reports starting the containment spray system before reactor vessel failure), it could set off an ex vessel steam explosion in the event of a fast bottom head failure of the reactor. Dropping a large mass of melted fuel into the standing water could set off a violent steam reaction that the containment structure can not cope with. More is written about this theory here along with Ian Goddard’s video explaining the process.
TEPCO admitted last year that the containment cap for unit 3 was failed and the unit leaks out through the reactor well. Both JNES and TEPCO analysis done in 2013 confirmed this failure.
There have been many reports of debris with high radiation, plutonium findings and other events that raised concerns that the radiation releases from Fukushima were far worse than originally admitted. Many of the early radiation release estimates were based on TEPCO’s early statements of the meltdowns. These early statements claimed much of the melted fuel was still retained in the reactor vessels and avoided admitting other now known issues like containment failures. How this new understanding would impact source term and release estimates is not currently known. A revisit of these early estimates might be in order.
Previous article on TEPCO admitting a total melt through http://wp.me/p1zinA-3xu
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